This invention relates generally to a process for preparing polymers having a low residual monomer content while retaining material properties such as flow, impact and color.
In a two-step embodiment to manufacture ABS in a typical emulsion process, a rubber latex is made in the first step. EPA 762693 discloses the first step of the emulsion process, a semi-batch process for the manufacture of diene rubber latex wherein a chain transfer agent such as t-ddm, is added upfront in the beginning of the reaction with the butadiene monomer and initiator. In order to get a sufficient low crosslink density in the rubber latex, high levels of chain transfer agents must be added with associated raw material costs and problems with reactor fouling as well as residual odor in final ABS products from the chain transfer agents.
In the second step of an emulsion process, acrylonitrile and styrene may be emulsion polymerized with the diene rubber latex to form an acrylonitrile-butadiene-styrene (ABS) graft polymer. The polymerization reaction is typically run to completion. However, sometimes even when the polymerization is substantially complete, undesired amounts of acrylonitrile monomer and/or styrene monomer dissolved or occluded in the polymer and in the reaction system water can still remain. The normal unit operations of stripping by vacuum or steam stripping the reaction system content do not remove all of this undesirable residual acrylonitrile and/or styrene monomer.
The importance of monomer conversion in ABS material has been recognized since its inception as a commercial material. The acute toxicity of acrylonitrile is well known and both butadiene and styrene monomers have been associated with health issues. From a manufacturing perspective, the full utilization of the monomers provides both for higher productivity and lower costs associated with recovery, recycle or disposal of the residual monomers. Also, manufacturers must abide by regulations that apply to their plant which place limits on allowable volatile material releases. In the marketplace, there is an increasing desire to deliver ABS material (e.g., pellets) with low levels of volatile materials.
There are several processes which are employed to achieve low residual monomer levels in the ABS product. In many cases, this is done by drying in ovens or fluid beds after the polymerized material is isolated. This xe2x80x98end-of-pipexe2x80x99 approach allows for excellent reduction of the residual monomers, but at the expense of both investment capital and operating costs to achieve the desired reduction. A potentially more effective approach entails the more complete conversion of the monomers during the polymerization reaction. Even within this avenue, several different methodologies have been used.
The use of additional monomers to aid in conversion has been employed both during the main reaction and at the end of the grafting reaction. For example, vinyl carboxylates have been used during the course of the reaction. Other patents describe the use of additional monomers at the end of the reaction including butadiene (U.S. Pat. No. 4,272,425), acrylonitrile (U.S. Pat. No. 4,822,858), n-vinyl mercaptans (German Patent No. DE 3,327,190), and methyl methacrylate (MMA) (U.S. Pat. No. 3,991,136xe2x80x94after polymerizing at least about 90% of monomer formulation). Most of the work with additional monomers do include some additional initiators also. U.S. Pat. No. 4,301,264 describes the use of a secondary initiator late in the reaction.
The problem with existing approaches has been that either the residual monomer levels are still higher than desirable or that a concomitant loss in other physical properties (e.g., color or impact strength) makes the resultant material unattractive as a commercial product.
As noted above, it is highly desired to lower emission of volatile hazardous organic components. As the conversion to ABS graft polymer is increased in such a polymerization to reduce residual monomers (which ultimately end up as hazardous volatile organic compounds) at the end of a graft polymerization, the rubber crosslink density of the graft polymer is also increased which adversely affects the impact, especially at low (sub zero) Fahrenheit temperatures.
In order to improve the low temperature impact, typically in the past either the graft molecular weight is increased and/or a higher molecular weight matrix styrene acrylonitrile (SAN) is used. However, either of these two approaches raise the melt viscosity of the blend, thus adversely affecting the flow and processability of the blend. Another approach used is to increase rubber level in the product formulation, which also reduces flow and reduce modulus.
In one embodiment of the invention, a semi-batch process for preparing low crosslink density diene rubber substrate is provided. The process includes adding an initial liquid batch to a reaction system, said liquid batch includes water, an emulsifier, and a diene monomer. The process further includes adding a liquid feed composition including butadiene, an initiator and a chain transfer agent to the reactor system, at or after the conclusion of the addition of the liquid batch to the reaction system. The continuous addition of the chain transfer agent allows for the preparation of a low density cross linked diene rubber substrate.
In another embodiment, a method of optimizing the flow and low temperature impact of ABS graft polymers is provided, which includes associating the rubber crosslink density of diene substrate used to make ABS graft polymer with the room temperature and low temperature impact strength of the ABS graft polymer made therefrom, and selecting the appropriate crosslink density diene substrate associated with the desired flow and low temperature impact of the ABS graft polymer.
In a third embodiment, a method of maintaining the flow-impact balance in an ABS graft polymer product while achieving higher conversion and reduced monomer emissions is provided. The method includes offsetting a crosslink density (% A) increase in the ABS graft polymer (due to increased conversion) by selecting a diene substrate of lower crosslink density (% A) to prepare the ABS graft polymer product.
In yet another embodiment of the present invention, an emulsion polymerization process for the preparation of polybutadiene grafted with styrene and acrylonitrile monomers in a reaction system is provided. The resultant product has a low end content of unreacted residual monomers. The process includes: a) charging the reaction system with a diene emulsion; b) adding to the reaction system, over a predetermined time, an optional initiator, acrylonitrile and styrene monomers; c) polymerizing the catalyzed reaction mixture of polybutadiene, styrene and acrylonitrile; and d) adding a third monomer and an optional initiator to the reaction mixture after the conversion rate of the monomers is about 98% or higher.
In another embodiment, an emulsion polymerization process for the preparation of ABS graft polymer having a low yellowness index is provided. The process includes maintaining an appropriate ratio of unreacted styrene to acrylonitrile monomers in the reaction system.